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(1) Sound energy is radiated uniformly in all directions from a small source at a rate of 1.2 w. (a) What is the intensity of the sound at a point 25 m from the source? (b) What is the relative intensity (the decibel measure) at this reception point?
Assuming no absorption of sound energy by the transmitting medium, the 1.2 w of sound power is associated with a spherical area having a radius of 25 m.
I = EQ \f(P,A) = EQ \f(P,4pr2) = EQ \f(1.2w,4p(2500cm)2) = 1.5x10-8 EQ \f(w,cm2)
b = 10 log EQ \f(I,Io) = 10 log EQ \f(1.5 x 10-8 w/cm2,10-16 w/cm2)
b= 10 log (1.5 x 108)
b = 10 (log 1.5 + log 108) = 10 (0.1761 + 8) = 82 db
type of soundintensity (db)threshold of hearing0whisper10-20very soft music30average residence40-50conversation60-70heavy street traffic70-80thunder110threshold of pain120jet engine170 (1) Sound energy is radiated uniformly in all directions from a small source at a rate of 1.2 w. (a) What is the intensity of the sound at a point 25 m from the source? (b) What is the relative intensity (the decibel measure) at this reception point?
What general principles should be followed when solving problems using the overhead projector or black/white board?
(1)
(2)(3)
(4)(5)
(6)
4.5. PAGE 3 Overheads
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